Mosquito dunks are a widely used biological method for controlling mosquito populations in standing water. They are typically small, beige, donut-shaped briquettes that float on the water surface and slowly release the active ingredient, Bacillus thuringiensis israelensis, or Bti. Mosquito dunks do expire, but this is a gradual loss of potency rather than a traditional chemical breakdown. The product remains safe for the environment and non-target organisms, simply becoming less effective over time.
The Active Ingredient and Shelf Life
The efficacy of mosquito dunks relies on Bti, a naturally occurring soil bacterium. This bacterium forms spores that contain an insecticidal protein crystal. When a mosquito larva ingests the spore and crystal, the alkaline conditions within its gut activate the protein, causing it to rupture the larval gut lining and kill the insect.
The product’s shelf life is directly related to the viability of these microscopic spores and their associated protein crystals. Manufacturers often state a shelf life of around two years from the date of production under optimal conditions. Some government-run mosquito control programs have reported stored dunks retaining effectiveness for up to eight years.
Storage Conditions That Reduce Effectiveness
The decline in Bti potency is accelerated by environmental factors that compromise the integrity of the spores. The greatest threat to the long-term effectiveness of stored mosquito dunks is exposure to high temperatures. Research indicates that prolonged exposure, especially over 24 consecutive hours, to temperatures exceeding 100 degrees Fahrenheit (38 degrees Celsius) can significantly reduce spore viability.
Direct sunlight is also detrimental because of its ultraviolet (UV) radiation component. UV light causes the Bti toxins to degrade, rapidly diminishing the product’s killing power. Additionally, exposure to excessive moisture or high humidity can prematurely activate the spores, leading to their eventual breakdown.
How to Ensure Maximum Efficacy
To maintain full potency, store mosquito dunks in a cool, dark, and dry environment. A basement, garage cabinet, or interior location that remains consistently below 100°F is ideal. Freezing temperatures do not harm the Bti spores or reduce efficacy, making cold storage acceptable.
The original packaging helps protect the dunks from humidity, but placing the briquettes in a sealed, airtight container provides an extra layer of defense against moisture. One full dunk is designed to treat up to 100 square feet of standing water for 30 days or more. For smaller water sources like birdbaths or rain barrels, break the dunk into smaller pieces for precise application. Consistent replacement every four weeks during the mosquito season ensures continuous control.